I'm just looking for general info on how long a tandem is--not for fitting purposes, just storage. Ours hasn't yet arrived and I'm terrible at estimating how long the ones we test-drove were, but since we don't have a garage I'm looking at outdoor storage sheds. (We already have a mountain bike in guest room and a road bike in the office, so we're running out of space!) Obviously there will be a range of sizes, but in general are we talking about 7 or 8 feet or so for a non-custom tandem? (It's a Burley Rumba, more specifically.)

Sorry; easy target. However, I would note that a tandem can usually be "stood" on it's rear wheel with the front wheel off and propped-up in the corner of a room. You must block the rear wheel so that it doesn't roll out and away from the wall and carefully set it in the corner with the the seats and handlebars learning against the back wall. The bottom of the bike (underside of the boom tube) ends up facing into the room, thus the rear tire is actually a foot or two away from the wall. The captain's saddle and handlebars / brake hoods are what rest against the wall behind the tandem, with either the left or right side of the captain's handlebars resting against the adjacent wall. I use this technique when we stay in hotels while on tours when our "ultimate tandem accessory/rack" (that would be our Suburban) is not available to us for bike storage. I use my helmet as a block as the rear tire fits right into the center cooling vent and the base of the helmet usually has a pretty good "bite" on hotel carpet (dont' go there). It looks weird when folks walk into the room, but it really is a great way to free up all but a 3' x 2' corner of a room. I put a towel over the the handlebars and a handkerchief over the saddle when the walls are painted instead of wallpapered to preclude leaving any smudges.

Sounds nuts, but it's something to consider whenever you are cramped for space and need a place to put a tandem so that it's out of the way. In fact, whenever we're in a crowded place where we need to walk the tandem I'll also rotate it up on the rear wheel (similar to how many of us learned how to move a personal bike through a sea of bicycles) and guide it around by the stoker's handlebars. Again, it looks bizarre but it keeps the tandem out of harms way, makes moving through a crowd a heck of a lot easier, and your stoker will always be able to find you. Getting it up and down does take a little care so as not to hit anyone or drop the bike: kinda like a clean-and-jerk movement.

I would note that a tandem can usually be "stood" on it's rear wheel with the front wheel off and propped-up in the corner of a room. You must block the rear wheel so that it doesn't roll out and away from the wall and carefully set it in the corner with the the seats and handlebars learning against the back wall. The bottom of the bike (underside of the boom tube) ends up facing into the room, thus the rear tire is actually a foot or two away from the wall. The captain's saddle and handlebars / brake hoods are what rest against the wall behind the tandem, with either the left or right side of the captain's handlebars resting against the adjacent wall.

Thanks for that great tip. We have 6 bikes in the living room and that little trick made room for one more...

Yep, that's one of the ways to do it!
There are also ready made wooden racks available to store bikes, two above each other; the handyman could easily make one like it.
Pulling the 'wheelie' bit with the tandem also works to get into some elevators that just don't seem to have adequate room. Have had some surprising looks when we do that!
Pedal on TWOgether!
Rudy and Kay